Phonon Anharmonicity and Phase Transitions in Perovskites
Lattice dynamics and phonon quasi-particles are commonly used to describe atomic vibrations in crystalline materials, typically within a harmonic approximation. However, anharmonicity is crucial to understand the thermal transport and thermal dynamics properties, as well as critical phenomena such as phase transitions. Using inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS) techniques, one can probe the phonon spectra functions, providing information about frequencies and linewidths at chosen momentum transfers, and as a function of temperature. Further, modern first-principles simulations and machine learning accelerated molecular dynamics simulations, enable the computation of anharmonicity and temperature dependent effects explicitly. By combining these experimental and computational approaches, this thesis investigates phonon anharmonicities in different perovskite systems. Anomalies in measured INS intensities of phonon are observed in SrTiO3 , upon cooling to approach this material’s ferroelectric quantum critical point. First-principles simulations including anharmonic effects enable us to quantitatively reproduce the experimental phonon S(Q, E), and its temperature dependence. It is found that changes in the eigenvectors of transverse acoustic and transverse optic modes lead to the anomalous INS intensity behaviors. Moreover, these changes were associated to the temperature dependent force-constants of the nearest Ti-O bond. A striking diffuse scattering signal of rods along the cubic Brillouin zone edges was found in CsPbBr3 in neutron/x-ray diffuse scattering experiments, forming a complex network in reciprocal space. Those diffuse rods reveal dynamic two-dimensional fluctuations of the system in real space. Using ab-initio molecular dynamics and temperature-dependent effective force-constants extracted up to third-order, we showed the strong diffuse signal arises from overdamped zone boundary acoustic modes with wave vectors connecting M and R points. These low-energy modes correspond to large amplitude PbBr6 octahedra rotations and dynamically modulate optoelectronic properties, through their coupling with Pb and Br derived band edge electronic states. In Cs2AgBiBr6 , diffuse signals are observed in neutron/x-ray diffuse scattering experiment, showing similarity to those observed in CsPbBr3 , implying that two-dimensional halide octahedra fluctuations likely exist in the broader halide perovskite family. Owing to the Brillouin zone folding as the unit cell doubles, the diffuse signals in Cs2AgBiBr6 is derived from damped low-energy optic modes connecting Brillouin zone-centers, as revealed by our inelastic neutron experiments and confirmed by our machine-learning accelerated molecular dynamics simulation. Further, a previously unreported low temperature phase transition was observed around 38 K, corresponding to the appearance of multiple superlattice peaks around wave vector (0.2,0.2,0). Our density functional theory (DFT) simulations on the tetragonal phase predict a soft phonon mode around this wavevector, providing insight into the phase transition mechanism. The long wavelength modulation corresponding to the superlattice indicates that the ground state structure likely contains hundreds of atoms. In the oxide perovskite KTa(1−x)NbxO3 (KTN, x = 0.35), correlated disorder from Nb and Ta off-centering is revealed via sheets of diffuse scattering intensity in reciprocal space, similar to previous observation in pure KNbO3 and BaTiO3. Our neutron scattering measurements of these diffuse sheets in KTN and a cross-correlation technique allow us to identify that they come from slow motions (energy ≤ 1 meV). Pure KTaO3 did not exhibit such quasi-elastic diffuse sheets on the other hand. Because of the correlated atomic disorder, selective phonon broadening is observed in inelastic neutron scattering experiments, with the zone-center ferroelectric mode and transverse acoustic modes polarized along h100i becoming damped. Our first-principles simulations and machine-learning molecular dynamics confirm that the disorder comes from correlated B-site (in a ABO3 system) off-centering in KTN. NbNiTe 2 was proposed to be a Weyl topological semimetal, while its crystal structure remains debated. Our inelastic x-ray scattering experiments reveal a zone-center soft mode in this compound, suggesting a second-order or weakly first-order phase transition at ∼373 K. Furthermore, our DFT simulations predicted a monoclinic ground state structure of this compound, which was later confirmed by single-crystal structure refinement.
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